The present invention relates to a method of forming a small gap using CMP (chemical mechanical polishing) and a method of manufacturing a lateral FED (field emission device) using the same.
In a device such as a sensor or a driver, generally, the greater electrostatic force is, the greater output signal is obtained. In the case of a FED, as electrostatic force becomes greater, its operating voltage is lowered, which enables its application to portable electronic appliances.
To increase the electrostatic force between electrodes, (1) the applied voltage should be increased; (2) area of the electrode should be increased; or (3) distance between the electrodes should be decreased.
The first method is not advisable because it needs the application of high voltage. And the electrostatic force is linearly proportional to the area of an electrode whereas it is inversely proportional to the square of distance between electrodes. Accordingly, the third method is the most effective one to obtain great electrostatic force.
To increase the electrostatic force, traditional MEMS (MicroElectroMechanical System) has used the second method (forming an electrode structure having a high aspect ratio) rather than the third method. It was not till recent days that the researches are concentrated on the third method.
As an example of prior art for reducing the distance between electrodes may be cited a process where comb structures of coarse teeth and comb structure of fine teeth are formed on a poly-silicon layer by means of thermal oxidation process, and then the fine teeth are located between the coarse teeth. However, this method requires additional process of locating the coarse teeth between the fine teeth, so the process itself is very difficult.
Another example is to form a comb-drive of a somewhat wide spacing, using poly-silicon, and then to deposit a poly-silicon layer thereon to decrease the wide spacing. However, this method exhibits low repeatability because only when the poly-silicon is evenly deposited on the underlying structure, the spacing between electrodes can become regular, and uniform electrostatic force can be obtained.
One example of application areas where a small gap is required is FED. FED is widely used in diverse areas such as flat panel displays, active elements of an integrated circuit, electron guns, microwave tubes, and sensors of various kinds.
The FED should have low operating voltage and uniform field emission characteristics. In the case of a lateral FED, to lower its operating voltage, it is desirable to reduce the spacing between electrodes. For this purpose, prior arts have used electrical stress or thermal stress. However, these methods have difficulty in making the spacing uniform though they can narrow spacing between electrodes. Therefore it is difficult to obtain uniform field emission characteristics by means of these methods.
Prior methods to fabricate a lateral FED may be divided as follows: First, to make a small gap between electrodes by means of electron beam lithography. A small gap may be formed with this method, which, however, has a defect that it entails too much process cost. Second, to apply heat of high temperature to poly-silicon, and utilize a small gap that is formed when poly-silicon is cut due to thermal stress. This method has a defect that a manufacturing process of high temperature is required and the repeatability is relatively low. Third, to pattern a Pd0 thin film, and utilize a small gap formed by electric stress. This method also has a defect that the repeatability is relatively low because it uses stress.
As stated above, although it is important to make a very small and uniform gap between electrodes, researches on it have not been satisfactory up to now.
Accordingly, it is an object of the present invention to provide a method for forming a small gap that can solve the aforementioned traditional problems through employing CMP, which is an entirely new method.
Another object of the present invention is to provide a method for fabricating a lateral FED having low operating voltage and uniform field emission characteristics through forming a probe with a sharp point, using the method of forming a small gap provided by the above object.